Graham Corporation Technical Article: An Introduction to Spike Ejectors

An Introduction to Spike Ejectors

May 2022

Greetings!

This article is going to focus on spike ejectors. Spike ejectors are a unique design often used in very specific applications. The aim of this article is to provide an introduction to spike ejectors and explain how they work and where they can provide additional benefit over a traditional ejector design.

Spike ejectors are used for short compression ranges. Unlike critical compression ejectors which are design for compression ranges that are more than 1.8:1, these sub-critical ejectors operate without a shockwave. In a sub-critical ejector design, the capacity of the ejector will change with its motive flow rate. In many design cases, the motive pressure is not a variable that can be controlled with great precision but the process conditions may still require a level of control. In order to control the amount of motive going into the spike ejector, a tapered rod is inserted into the motive nozzle throat. The rod is then moved into or retracted from the nozzle to change the active cross sectional area inside the nozzle throat. By manipulating the position of this spike rod the amount of motive flow into the ejector can be controlled, even at a constant motive pressure.

Control

The primary reason to use a spike ejector is for control. This type of ejector can be used in two primary types of control schemes. The control scheme used for a specific application is going to be dependent on that application’s needs. The control arrangements are either suction control or discharge control, see a description of each below.

Suction Control – With this type of a control scheme, the position of the spike rod is going to be manipulated to control the ejector’s suction pressure. Feedback from a pressure transmitter on the suction line is used for this control scheme. If a deeper suction pressure is required, the rod is retracted allowing more motive flow. If a poorer suction pressure is required, the rod is inserted causing the ejector to flow less motive. In this way, the spike ejector can be used to control the suction pressure with a high degree of accuracy.

Discharge Control – This control arrangement is used when trying to maintain the discharge pressure of the ejector. While the ejector does not directly control the discharge pressure, manipulating the spike rod position will impact the overall discharge flow rate and that flow rate will impact the ejector’s discharge pressure. By using feedback from a discharge pressure transmitter, the position of the spike can be moved which impacts the overall flow rate. If a higher discharge pressure is needed, the spike rod is retracted allowing for more motive flow. If a lower discharge pressure is needed, the spike rod is inserted reducing the flow rate.

Common Applications

There are some specific applications where a spike ejector is often utilized, although the examples outlined below are only some of the more common uses for this type of equipment.

Steam Pressure Recovery – One common application for spike ejector is steam recovery. One of the benefits of a spike ejector is that it can recover low pressure steam that may currently be sent to waste. The lower pressure steam may result from a process that uses medium pressure steam. In this case, the plant high pressure steam source is used to compress the low pressure "waste" steam so it can be resent into the medium pressure network.

Example: Medium pressure steam is used in a heater at 150 psig and is output at 100 psig. The plant can use their high pressure steam source at 600 psig in a spike ejector to compress the waste steam back to 150 psig, resulting in an overall energy savings.

Process Heat Control – If a process requires a certain amount of heat and that heat is being introduced from an external steam source, a process may elect to use a spike ejector. In this application, the amount of heat energy is going to be directly related to the steam flow. A spike ejector can be used to control that flow into a process. While a direct steam source could be used, passing that steam through a spike ejector can allow that ejector to compress a secondary source of steam or do work on a secondary process.

Example: If a process runs at 150 psig, a 100 psig steam source can’t be used to heat the process as its pressure is too low. Using a spike ejector, a 300 psig steam source could compress a 100 psig steam header so that the make-up of flow is a mixture of flows from a 300 psig header and 100 psig header. In doing this less 300 psig steam would be needed when compared to just injecting 300 lb steam.

While these are just a couple of common applications, many other applications exist and not all of them can be reviewed in detail in this article. Spike ejectors have a lot to offer processes that require control and can often be used to gain efficiencies within a plant. If you have an application where you think a spike ejector would be appropriate please reach out to Graham.